xfs: fix up asserts in xfs_iflush_fork

Remove asserts in xfs_iflush_fork that would call xfs_iext_get_ext
with a potentially invalid extent buffer index.

Based on an earlier patch from Lachlan McIlroy.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Lachlan McIlroy <lmcilroy@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_inode.c

@@ -2557,12 +2557,9 @@ xfs_iflush_fork(
 	case XFS_DINODE_FMT_EXTENTS:
 		ASSERT((ifp->if_flags & XFS_IFEXTENTS) ||
 		       !(iip->ili_format.ilf_fields & extflag[whichfork]));
-		ASSERT((xfs_iext_get_ext(ifp, 0) != NULL) ||
-			(ifp->if_bytes == 0));
-		ASSERT((xfs_iext_get_ext(ifp, 0) == NULL) ||
-			(ifp->if_bytes > 0));
 		if ((iip->ili_format.ilf_fields & extflag[whichfork]) &&
 		    (ifp->if_bytes > 0)) {
+			ASSERT(xfs_iext_get_ext(ifp, 0));
 			ASSERT(XFS_IFORK_NEXTENTS(ip, whichfork) > 0);
 			(void)xfs_iextents_copy(ip, (xfs_bmbt_rec_t *)cp,
 				whichfork);